Wikipedia says this:"Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens) is often used in film and digital SLR cameras. The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple rangefinder with a base identical to the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement."

which leaves a lot to be desired. (My Rebel has multiple focus points, how does that match up with "a simple range finder with a base identical to the lens diameter") Bob, do you have a better explanation for this, or can you point me to one? Might make a good article...

I think the easiest way to look at it is that it works somewhat like the split-image focusing aid that many manual SLRs had in the center of the focusing screen.

If you want a more detailed technical explanation, you can find one in Applied Photographic Optics by Sidney F. Ray. Here's a link to the section of that book that deals with AF - Applied Photographic Optics